Background
Serotonin 1A (5-HT1A) receptors are implicated in the pathogenesis of several psychiatric and neurodegenerative disorders motivating the development of suitable radiotracers for in vivo positron emission tomography (PET) neuroimaging. The gold standard PET imaging agent for this target is [carbonyl-11C]WAY-100635, labeled via a technically challenging multi-step reaction that has limited its widespread use. While several antagonist and agonist-based PET radiotracers for 5-HT 1A receptors have been developed, their clinical translation has been hindered by methodological challenges and/or and non-specific binding. As a result, there is continued interest in the development of new and more selective 5-HT1A PET tracers having a relatively easier and reliable radiosynthesis process for routine production and with favorable metabolism to facilitate tracer-kinetic modeling. The
Conclusion
Despite efficient radiolabeling, results from PET imaging experiments limit the application of [11C]3 for in vivo quantification of 5-HT1A receptors. Nevertheless, derivatives of compound 3 may provide a scaffold for alternative PET radiotracers with improved selectivity for 5-HT 1A receptors or α1-adrenergic receptors.
Results
Syntheses of 1-3 and corresponding precursors for radiolabeling were achieved from isonicotinic, picolinic acid or picolino nitrile. In vitro binding studies demonstrated nanomolar affinity of the compounds for 5-HT1A receptors. Binding of 1-3 for other biogenic amines, neurotransmitter receptors, and transporters was negligible with the exception of moderate affinities for α1-adrenergic receptors (4-6-fold less potent than that for 5-HT1A receptor). Radioligands [11C]1-3 were efficiently prepared by 11C-O-methylation of the corresponding phenolic precursor in non-decay corrected radiochemical yields of 7-11% with > 99% chemical and radiochemical purities. Dynamic PET studies in rats demonstrated negligible brain uptake of [11C]1 and [11C]2. In contrast, significant brain uptake of [11C]3 was observed with an early peak SUV of 4-5. However, [11C]3 displayed significant off-target binding attributed to α1-adrenergic receptors based on regional distribution (thalamus>hippocampus) and blocking studies.